This invention relates to method and apparatus for measuring electrical parameters of geological core samples, and more specifically to nondestructive test electrodes for use in obtaining the descriptive data, to a test apparatus incorporating the test electrodes, and to a method to obtain data indicative of frontal instabilities associated with the movement of immiscible substances through porous media by measuring changes in electrical characteristics of the core sample.
A number of processes are known for enhanced recovery of oil from underground reservoirs. such recovery processes typically involve an injection of a specified material into the reservoir at an injection well and recovery of the oil in a producing well. The various processes utilized in such enhanced recovery of oil include water flooding, surfactant flooding, gas injection (e.g., carbon dioxide) and application of heat, for example. The heat applied to the stored hydrocarbons may be derived from combustion of a portion of the in-place oil (in situ combustion) or, alternatively, may be externally generated and provided to the reservoir by a heat carrier such as steam or hot water injected into the underground formation.
In each of the above processes, various phase fronts are displaced within the underground reservoir. Such phase fronts include, for example, interfaces between the injected materials and the underground hydrocarbons, interfaces between various combustion products of the underground hydrocarbons, temperature fronts and flame fronts.
In order to recover maximal quantities of oil it is desirable to know the displacement of the various underground fronts and the direction of travel thereof. Such knowledge enables the drilling of production wells at appropriate sites, as opposed to using arbitrary geometrical patterns for injection and production wells. Additionally, information pertinent to the displacement of the various fronts permits drilling of two types of additional wells to increase oil recovery. Injection wells may be drilled for injection of controlling substances to direct oil towards production wells. Additionally, new production wells may be drilled at the location of the fronts or at a determined forthcoming location thereof.
Accordingly, the prior art has developed techniques for analysis of the response of reconstituted geological core samples to various ones of the injection and flooding processes. Laboratory equipment has been developed, for example, to provide physical and chemical analysis of various combustion products of a core sample, simulating an in situ or underground combustion recovery process, as described in "Secondary and Tertiary Oil Recovery Processes", Interstate Oil Compact Commission, Oklahoma City, Okla., September 1974 at pages 100-101.
A different combustion tube system is described in Harding et al., "Adiabatic Combustion Tube Evaluation of In Situ Processes for Oil Sand", 26th Canadian Chemical Engineering Conference, Toronto, Canada (October 1976) and in Moore et al., "Observed In Situ Combustion Phenomena", preprint (1981). The described structure may be used to measure electrical characteristics of a reconstituted core sample subject to conditions simulating in situ combustion, water flood, steam flood, chemical flood, or any process where an agent is injected to improve recovery. The electrical characteristics of the sample, and the changes therein, are useful in determining the progress of the various fronts associated with enhanced oil recovery processes.
For example, it is known that substantial changes in resistivity occur in the reservoir materials as a consequence of passage of the various fronts therethrough. Such data may be detected in the field by electromagnetic induction geophysical prospecting techniques, such as the controlled source audio frequency magnetotelluric process described in Ostrander, "CSAMT--Application and Advantage", preprint of paper presented at 87th Annual Northwest Mining Association Convention, Spokane, Wash., Dec. 4, 1981, or the method described in Ginsburgh et al., U.S. Pat. No. 4,210,868. The interpretation of the field data requires knowledge of changes in the electrical characteristics of the specific formation subject to the enhanced recovery process.
There is thus a need in the prior art to provide laboratory equipment for providing accurate data representative of a simulated in situ combustion (or other enhanced oil recovery) process before, during and after the passage of a phase front therein.
Further, in order accurately to interpret prospecting data during an enhanced recovery process, it is necessary to be able to analyze frontal instabilities dynamically in the laboratory. The prior laboratory test equipment, subject to destruction and corrosion in response to various phases of the recovery process, cannot provide accurate data representative of frontal instabilities, temporary aberrations or similar data. There is thus a further need in the prior art for a method and apparatus for dynamic measurement of such frontal instabilities.